The most commonly used rotary speed changers, i.e., worm and spur gear trains, are inefficient and involve substantial friction and wear, especially at high input or output speeds such as 10,000 rpm. For greater efficiency and/or longer life, more sophisticated and expensive devices have been employed such as the internal gear sets shown in U.S. Pat. Nos. 3,367,239 (Takagi) and Re. 26,383 (Huber).
Highly efficient speed changing is provided by the internal gear set illustrated in FIGS. 1 and 2 of my U.S. Pat. No. 3,979,167. At an input speed of 9000 rpm, the inner gear member pumps fluids at 1000 rpm, a speed at which shaft seals will last indefinitely. Although this internal gear set is less expensive than those of the above-cited U.S. patents, there has been a continuing need for an efficient rotary speed changer of even lower cost.
A speed changer which bears a superficial similarity to that of the present invention is shown in U.S. Pat. No. 2,170,951 (Perry). That speed changer is exceedingly expensive, necessarily having large numbers of involute teeth. Because all teeth are not in continuous mesh, there is considerable meshing noise.
It may occur to one skilled in the art to attempt to replace the gears of Perry with the gears shown in FIG. 1A U.S. Pat. No. 2,209,201 (Hill) in spite of the fact that Hill's teeth are rather sharp and hence can break under the very high sideloading pressures that can be encountered at high speed-reduction ratios. In an internal gear set made as shown in Hill's FIG. 1A, the ratio of the root diameter of the inner gear element to the eccentricity is 6.57 to 1. That ratio for the inner gear element of Hill's FIG. IA appears to approach 7 to 1. Such a small ratio does not permit the inner gear to be formed with cycloid holes without undue weakening of either or both the inner gear and the cycloid pins.